This invention relates generally to electricity-producing batteries and their construction. More particularly, this invention relates to electrode and electrolyte compositions for lithium-ion and lithium polymer batteries that improve capacity-fade resistance characteristics of the battery.
The need for higher energy and power is becoming increasingly valuable in most portable applications as manufacturers continue to incorporate more features in their products. End users continue to also demand longer operation times as well as longer life. These demands and features place a very strong requirement on the storage energy and cycle life characteristics of the associated energy source.
Lithium-based battery cells are an attractive energy source for portable applications, due in part to their ability to provide relatively high energies and long cycle life. Lithium is the lightest of all the metals, with a high electrochemical potential, thus providing high energy densities. Rechargeable batteries using lithium as the electrochemical material are capable of providing higher energy to weight ratios than those using other chemistries.
The voltage of these cells, which corresponds to the total energy capacity, depends on the choice of the anode and cathode couples used in the cell. High voltage cathode materials include, among, others, LixMnOy, LixCoOy, and LixNiOy. Of these, the nickel based material, particularly LiNiO2, has the highest capacity, and as a result has become a focus area for the enhancement of lithium cell energy. While cells incorporating this material yield higher energy, assessing all the energy leads to material instability that in turn results in poor cycle life characteristics of the cells. There is therefore the need to provide LiNiO2 based lithium cells with high energy (capacity and voltage) and better cycle life characteristics.
Several variations of the lithium cell design exist today, one of which is the lithium-ion polymer construction. Lithium polymer battery cells are conventionally and broadly described as xe2x80x9crechargeable battery cells that are constructed with either a solid or xe2x80x98gelledxe2x80x99 electrolytexe2x80x9d. The lithium polymer battery offers some design advantages, including reduced thickness, over conventional (non polymer) designs. Also, in the case where the electrolyte is completely solid, it can be argued that the cells could avoid the deleterious effects of high flammability due to the presence of liquid electrolyte in non-solid systems.
One type of a Li-polymer battery is a polyvinylidene fluoride (PVDF) gel electrolyte cell. The PVDF is placed on a separator between the electrodes, and then the battery is subjected to a specified temperature such that the PVDF forms a gel with liquid electrolyte. The gelled PVDF serves as a xe2x80x9cgluexe2x80x9d to hold all the components of the battery together. Even though the Li-polymer battery with a PVDF gel electrolyte has a manufacturing advantage when compared to other polymer designs, the battery suffers from rapid capacity-fade when LiNiO2 is used as the cathode material. In other words, after repeated charges and discharges, the overall capacity of the Li-polymer battery to hold an adequate charge fades. This quality is disadvantageous in a battery because the battery will very quickly cease to provide a functional charge to a host device, such as a cellular telephone.
Accordingly, the existing Li-ion and Li-polymer batteries and electric cells that have a greater ease of manufacture suffer from a rapid capacity fade in the storable charge load. There is thus a need for an improved electrolyte composition.